Re: Very low disk performance on 5.x

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 05/02/05

  • Next message: Arne : "Re: Very low disk performance on 5.x" 
    To: Eric Anderson <anderson@centtech.com>
Date: Mon, 02 May 2005 15:54:59 +0200

    In message <42762AD8.1020607@centtech.com>, Eric Anderson writes:

    >If I write a 10GB file to disk (RAID array has 1GB cache, system has 1GB
    >memory), then I should definitely see better read performance reading
    >that same file back to /dev/null than writing it, right?

    Nope, quite the contrary: you will get much better write performance
    because your write operation finishes the moment it has hit the cache,
    but your read request only comes fast if it happens to be in the cache.

    With a 1GB cache and a 10GB file, this clearly will not be the case
    unless some kind of very good read-ahead happens in the RAID5 unit. 

    -- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.
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  • Next message: Arne : "Re: Very low disk performance on 5.x"